East Asian monsoon precipitation decrease during Plio-Pleistocene transition revealed by changes in the chemical weathering intensity of Red Clay and loess-paleosol

Abstract

East Asian Monsoon variations during the Plio-Pleistocene transition have attracted much scientific interest, but they are still not well understood. We analysed the trace and major element concentrations of the Red Clay and loess-paleosol sequence of the XSC section, on the southern Chinese Loess Plateau (CLP), over the time interval of 4–2 Ma. From the results of geochemical and mineralogical analysis, we conclude that the Na/K ratio can be used to reflect the weathering intensity of plagioclase. Grain- size effects on the Na/K ratio were minimized based on the analysis of multiple size fractions. The Na/K ratios show similar trends between different grain-size fractions during the Pliocene-Pleistocene transition. Additionally, the Na/K ratio for various Holocene eolian sediments is well correlated with the regional mean annual precipitation (MAP). Based on this observation, we relate the Na/K ratio to East Asian Summer Monsoon (EASM) precipitation and propose that the EASM is the primary control of the post-depositional weathering intensity. The Na/K record of the EASM precipitation, as inferred from the Na/K record of the XSC section, progressively weakened and increased in amplitude between 4 and 2 Ma, indicating that the climate on the monsoonal margin evolved from warm and moist during the Late Pliocene to dry and cold during the Early Pleistocene. Our results support the hypothesis that global cooling was responsible for driving the decrease in EASM-derived rainfall via the migration of the intertropical convergence zone (ITCZ) and the associated monsoon precipitation belt during the Plio-Pleistocene transition.